I am indeed worried quite a bit about my wife losing the best friendship she's had in years. This person has been very good for her, in a lot of ways.
The innocence factor is more an issue for my wife than for myself. I never saw myself as innocent, even when I was; I had waaay too much of a sex drive in me from the onset of puberty onward to not accept that sexuality was a part of who I was as a person. But her . . . I had to teach her what sexuality was, and she still doesn't see herself as a really sexual creature (though that is actually a big part of her, and of how she and I interact & bond).
A related concern is that this will go south in a way that makes my wife feel ashamed for having thought about it in the first place. Having to reconsider what "marriage" means, to reconsider a basic part of our identity, this far into the game . . . is a lot of hard work. To have to do that, for something that might not even get off the ground, when she has no intention of broadening her horizons like this for anyone BUT the one person she developed feelings for after years of friendship, would be upsetting to her.
And yes, there's a sporadic, niggling, irrational fear that even opening this door means our marriage will end horribly within a year or whatever. I'm normally not afraid of that, because we've weathered a lot of storms, but the vast majority of the problems we've faced have been external challenges (job loss, problems with the kids, etc). This is a new kind of internal problem and reassessment for us.